Electrodeposition of silver



Patented Apr. 18, 1950 UNITED STATES PATENT OFFICE.

No Drawing. Applicationflctober 1944, Serial No. 560,351

1 Claim. 1

This invention relates to the electrodeposition of silver, and more particularly to improved electrodeposits of silver on metals and on metallic conducting surfaces on electrically non-con ducting materials.

An object of the invention is to provide for the electrodeposition of silver from a bath in which the silver ion concentration is maintained at a low value while at the same time a large quantity of silver is maintained in the solution in the form of complex ions and is readily available for conversion to free silver ions as the silver ions are used in the electroplating. Another object is to provide an electrodeposit of silver in which a mat surface is produced and in which deposits of desired ductility are obtained. A further object is to provide for the electrodeposit of silver by a process which possesses high electrical efficiency. Yet another object is to provide for the electrodeposition of silver on metals and also on conducting surfaces on electrically non-conducting materials, and particularly to provide for satisfactory adhesion of the electrodeposit of silver to the conducting surface of a nonconducting material.

Another object of the invention is to provide a bath for the electrodeposition of silver which is easily and readily prepared from materials which are economical and readily available commercially, and which bath avoids the disadvantages of the highly poisonous characteristics of cyanide baths. A further object is to provide for the electrodeposition of silver upon aluminum and magnesium and for a bath or solution which itself does not attack the aluminum or magnesium which is to be plated.

The invention contemplates producing improved electrodeposits of silver from a pyrophosphate solution. Silver pyrophosphate is insoluble in water and may not be used alone as a source of silver electrodeposition. However, in accordance with the present invention, silver pyrophosphate is used in combination with ammonium ion in the form of a complex to provide solubility and also to furnish the ions of silver in the solution in the desired concentration. Although the concentration of free silver ions within the solution is relatively low and the electrodeposit therefore possesses desirable characteristics as to grain and crystalline structure, the large quantity of silver present in the solution in the form of the silver ammonium pyro phosphate complex makes available for the electrodeposit a large quantity of silver.

The silver may be introduced into the solution in the form of various salts thereof. It may be introduced directly as the pyrophosphate or, if desired, in the form of other salts, such as the nitrate and sulfate. The pyrophosphate may also be introduced in any of various forms. For example, the pyrophosphate may be introducedin the form of a salt of" an alkali metal, such as sodium, or in the form of silver pyrophosphate. If silver pyrophosphate is used in making'up the solution, it is preferred that an excess of pyro pho'sphateionbe added by means of the addition of an alkali metal pyrophosphate, such as sodium pyrophosphate. If sodium pyrophosphate is mixed witha silver salt, such as silver nitrate, the quantity of pyrophosphate ion should preferably be in excess of the stoichiometrical quantity. The content of silver pyrophosphate in the solution is between 2% and 5% of the $011- tion.

The bath should contain a quantity of arm mcniunr ion in excess of that required for com bination with the silver pyrophosphate in the form of the complex. Since two ammonium ions combine with each silver ion in the complex, two gram equivalents of ammonium ion should be usiedto each gram equivalent of silver ion. In addition, a substantial excess of ammonium ion over and above this quantity has been found to be helpful. The ammonium ion may be introduced in the form of ammonium hydroxide or in the form of an ammonium salt, such as ammonium sulfate.

Although in the solution containing the complex only a portion of the silver and the arm-- monium is present in the form of free ions, the relationship between them will be referred to and considered herein as a relationship between the quantities of ions since upon removal of the free ions the complex disassociates or ionizes to furnish additional free ions. Accoringly, the solution is referred to as one containing in excess of two ammonium ions to each silver ion.

The bath preferably has a pH of between 8.5 and 9.7 and, for example, may have a pH of approximately 9.0. The hydrogen ion concentration should be controlled since if the pH of the solution is too low a precipitate will form and the electrodeposits of silver tend to be too crystalline. On the other hand, if the pH is too high, a precipitate also forms.

The characteristics of the bath are improved by the presence of ammonium sulfate or other ammonium salt in addition to the quantity of ammonium ion added for the purpose of forming the complex. Preferably, in the neighborhood of 4% to 12% by weight of ammonium sulfate with respect to the solution may be added. The animonium sulfate has been found to improve the ductility and increase the smoothness of the surface of the silver deposit which is formed.

The electroplating process may be carried out by passing an electrical current through the silver ammonium pyrophosphate bath, with the material which is to be plated being the cathode within the bath. Silver anodes may be used so 3 that the bath is replenished as it is operated. In addition, if desired, the silver anodes may be supplemented with carbon or steel anodes to prevent excessive accumulation of silver within the bath. On the other hand, if silver anodes are not used, then the silver salt within the bath must be replenished as the electrodeposition is carried.

out. A current density of 70 amperes per square foot and a voltage of 4.5 have been found to be suitable in the operation of the bath, but considerable variation in current conditions may be made without affecting the desirable characteristics of the deposit.

When the silver is to be applied to a dielectric or electrically non-conducting material, this material should be pretreated to apply a conducting surface thereto. The pretreatment may consist of the chemical deposition of silver in any suitable manner. The dielectric material bearing the chemically deposited silver conductor coating may be electroplated in the same manner as is a metal. Preferably, however, the concentration of ammonium sulfate or other ammonium salt in the bath is maintained below the concentration which is desirable for the electroplating of metals since otherwise the presence of the ammonium sulfate tends to interfere with obtaining desired characteristics of adhesion to the conducting surface of the electrically non-conducting or dielectric material. In the plating of dielectrics, the quantity of ammonium sulfate added to the solution is preferably in the neighborhood of 5% and not in excess of 8%.

In the plating of metals which are higher than silver in the electromotive series and therefore tend to displace silver in solution, it is preferred that a flash or prestrike coating of silver be applied to the material from a bath of low silver ion concentration prior to the application of the electrodeposit. This flash coating of silver is conventional in the electrodeposition of silver and any suitable flash coating may be used. This flash coating is desirable in the case of the plating of such metals as copper, lead, tin or iron, but is not necessary in the case of anodized aluminum and magnesium. These latter metals in the anodized state do not react with the silver ammonium pyrophosphate solution to produce a chemical deposit of silver, and the problem of flash coating therefore does not arise.

Specific examples of silver baths or solutions prepared in accordance with the invention are as follows:

Silver nitrate grams 30-475 Sodium pyrophosphate (N2.4P2O7.10H2O) do -40 Ammonium sulfate do 80 Ammonium hydroxide (specific gravity .90) 00-- 40 All of the above proportions are with respect to one liter of solution and the pH-oi the solution should be approximately 9.0.

Silver pyrophosphate grarns 26-42 Ammonium sulfate do 120 Ammonium hydroxide (specific gravity .90) cc 40 Sodium phyrophosphate grams 20 All of the above proportions are by weight with respect to one liter of solution and'the pH of the bath may be approximately 9.0. The above bath because of the large quantity of ammonium sulfate present is particularly adapted for the plating of metals and is not as Well adapted for the plating of dielectric materials.

III

Silver sulfate grams 28-45 Sodium pyrophosphate do 20-40 Ammonium sulfate do '70 Ammonium hydroxide (specific gravity .90) cc 40 All of the above proportions are by weight with respect to one liter of solution and the pH of the solution is preferably approximately 9.0.

The electrodeposition of silver in accordance with the invention has been found to produce a mat surface of high ductility and at the same time to be a highly electrically efficient process. The silver electrodeposit possesses desired characteristics of adhesion to both metals and metallic conducting surfaces on electrically non-conducting materials. The solution itself does not attack anodized aluminum and magnesium and is particularly adapted for the electrodeposition of silver on these materials. The disadvantages of the highly poisonous characteristics of cyanide baths are avoided. By reason of the formation of the silver ammonium pyrophosphate complex, the free ions of silver in the solution are at a relatively low concentration, but the quantity of silver in solution in the form of a complex is sufficient to provide ready availability of a large quantity of silver.

Although the invention has been described in connection with certain specific embodiments, it will be apparent that modifications and changes may be made without departing from the spirit and scope of the invention.

I claim:

A process for the electrodeposition of silver consisting of introducing the material to be plated into an aqueous solution consisting of between 2% and 5% by weight of silver pyrophosphate, an excess of pyrophosphate ion, in excess of two ammonium ions to each silver ion, and from 4% to 12% by weight of ammonium sulfate, the solution having a pH between 8.5 and 9.7, and passing an electrical current through the solution and to the material as the cathode.

EWALD H. MCCOY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 744,170 Darlay Nov. 1'7, 1903 923,864 Levy June 8, 1909 OTHER REFERENCES Hughes et 2.1., The Character of Silver Deposits from Various Electrolytes, The Journal of the American Chemical Society, vol. 32, pages 1571-6,Dec. (1910).

Zeitschrift fur Analytische Chemie, 28, page 592 (1889).

Mellor, Treatise on Inorganic and Theoretical Chemistry, vol. III, pages 487488, Longmans, Green and Co. Ltd. (1928).

Smith, The Electrolysis of Metallic Phosphates in Acid Solution, American Chemical Journal, vol. 12, page 335 (1890). 

